The present invention relates to a method of and a mechanism for feeding and positioning a sheet, and more particularly to a method of and a mechanism for feeding and positioning a sheet such as a stimulable phosphor sheet, a photographic photosensitive sheet, or the like in order to read or reproduce a recorded image from or on the sheet, by gripping the sheet with a pair of rollers, then spacing the rollers from each other to free the sheet therebetween, positioning the sheet in a prescribed position with a positioning means, and then gripping the positioned sheet again with the rollers to feed the sheet from the prescribed position for accurately reading or reproducing the image from the sheet.
There has recently been known a radiation image information recording and reproducing system for producing the radiation-transmitted image of an object using a stimulable phosphor material capable of emitting light upon exposure to stimulating rays. When a certain phosphor is exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, the phosphor stores a part of the energy of the radiation. When the phosphor exposed to the radiation is subsequently exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy of the radiation. The phosphor exhibiting such a property is referred to as a "stimulable phosphor".
In the radiation image recording and reproducing system employing such a stimulable phosphor, the radiation image information of an object such as a human body is stored in a sheet having a layer of stimulable phosphor, and then the stimulable phosphor sheet is scanned with stimulating rays such as a laser beam to cause the stimulable phosphor sheet to emit light representative of the radiation image. The emitted light is then photoelectrically detected to produce an image information signal that is electrically processed for generating image information which is recorded as a visible image on a recording medium such as a photosensitive material or displayed as a visible image on a CRT or the like.
The radiation image recorded on the stimulable phosphor sheet is read in the radiation image recording and reproducing system as follows:
The radiation image recording and reproducing system includes an image reader for two-dimensionally scanning the stimulable phosphor sheet with a light beam such as a laser beam to cause the stimulable phosphor sheet to emit light, and detecting the light in time series with a light detector such as a photomultiplier to obtain image information. The stimulable phosphor sheet is usually two-dimensionally scanned by deflecting the light beam and applying the deflected light beam to the stimulable phosphor sheet in a main scanning direction, and simultaneously feeding the stimulable phosphor sheet mechanically on a conveyor belt o the like in an auxiliary scanning direction substantially normal to the main scanning direction.
When reading a radiation image from the stimulable phosphor sheet, it is not ensured that the stimulable phosphor sheet will be positioned in a predetermined position at all times. That is, while the stimulable phosphor sheet is being fed by the conveyor means from a cassette or a magazine, it may be positionally displaced particularly in the transverse direction. If the stimulable phosphor sheet is transversely displaced in position, the radiation image recorded on the stimulable phosphor sheet may not accurately be read from the sheet. To avoid such a problem, any positional displacement which has taken place in the transverse direction should be corrected to position the stimulable phosphor sheet accurately in a desired position when the stimulable phosphor sheet is subjected to an image reading process.
However, where a plurality of roller pairs are employed as feed means for feeding the stimulable phosphor sheet, since the stimulable phosphor sheet remains gripped by the rollers, it is impossible to eliminate any undesirable transverse positional displacement or error of the stimulable phosphor sheet.